I was dabbling with world building as far back as elementary school if not preschool. I was always creating fantastic stories and settings for them to take place.
During high school I begun building a classic fantasy world setting for some of my stories, but also for rpg campaigns. That world has grown with me throught he years and gone through a lot of changes as I matured. I'm now it the process of writing a novel set in this world with the intent to send it in to publishers this year. This means that I feel a lot more preassure now to make sure the world feels believable and immersive. Maybe I can get some pointers in here when I stumble. This is my drive for joining.
I've also been building a post apocalyptic setting for some sci fi stories I'm working on sporadically.
My most classic mistake during early world building: making a tribe of people that ride on carnivorous mounts in a dessert setting. ...and then I did the maths, and then I wept.